This invention relates generally to a tandem drive axle set and, more particularly, to an improved bearing configuration for an inter-axle differential assembly for a tandem drive axle set.
A tandem drive axle set is used to distribute rotational power from a driveline input to a set of forward and rear wheels through a forward drive axle assembly and a rear drive axle assembly, respectively. Traditionally, the tandem drive axle set is designed such that the forward drive axle assembly has a ring gear and a pinion gear set that is a mirror image of the rear drive axle assembly ring gear and pinion gear set. Usually, the forward drive axle assembly has a right-hand pinion gear while the rear drive axle assembly has a left-hand pinion gear. It is necessary that the forward and rear drive axle assemblies be mirror images of each other because, traditionally, the forward drive axle assembly has included a set of helical gears that are used to transfer half of the rotational power from an inter-axle differential to the forward drive axle assembly pinion and ring gear set. That is, the forward and rear drives require distinct parts resulting in axle component proliferation, which is undesirable.
The inter-axle differential receives rotational input from the driveline of the vehicle. In such a design, the pinion gear of the forward drive axle assembly rotates in the opposite direction to that of the inter-axle differential. The inter-axle differential transmits the other half of its input to a through shaft, which sends the input back to the rear drive axle assembly. In the rear drive axle assembly the pinion gear rotates in the same direction as the inter-axle differential. Because the helical gears are necessary in the forward drive axle assembly, the axis of the input to the forward drive axle assembly is offset from the pinion gear axis in the forward drive axle assembly by the centerline-to-centerline distance of the helical gears. Therefore, the output of the forward drive axle assembly is on the same axis as the input while the input of the rear drive axle assembly is on the same axis as the forward drive axle pinion gear.
This difference in axis height between the forward axle output to the rear axle input requires different axle pinion angles to be utilized in order to set the driveline angles in the u-joints used in the driveline. Setting and maintaining the driveline angles is difficult. When the driveline angles at the u-joints are not the same it creates adverse torsional loading and vibrations in the drivetrain assembly. Such torsional loading and vibrations can lead to premature failure of the drivetrain assembly. Even when the driveline angles are properly set at the factory, the air-ride suspensions commonly found in heavy duty trucks can alter the driveline working angles in an adverse manner.
One solution is to utilize a hollow pinion gear in the forward drive axle assembly as described in U.S. Pat. No. 6,200,240 assigned to the assignee of the present invention. The through shaft extends from the inter-axle differential and through the hollow pinion gear. The hollow pinion gear drives the main differential in the forward drive axle assembly. The through shaft extends toward the rear drive axle assembly which utilizes a traditional pinion gear to drive a rear differential. This configuration allows a common axis to be shared by the input to the forward drive axle assembly and the input to the rear drive axle assembly, i.e. the hollow pinion gear and the rear pinion gear have a common axis.
One disadvantage with this configuration is that multiple bearings are required to support the inter-axle differential and additional bearings are required to support the hollow pinion gear. Each bearing represents a potential failure mode in the drive axle assembly. Additionally, each bearing requires a significant amount of packaging space within the drive axle assembly, which reduces available packaging space for other axle components.
Another disadvantage with the axle assembly described in U.S. Pat. No. 6,200,240 is that certain applications cannot be configured to have a common axis for the forward and rear pinions. However, it is still advantageous to reduce the vertical height difference between the forward and rear pinions as much as possible.
Thus, it is desirable to provide an improved tandem axle set configuration with fewer bearings but which still provides a common axis to be shared by the input to the forward drive axle assembly and the input to the rear drive axle assembly or which provides a significantly reduced vertical height between the pinion inputs. The improved bearing configuration should be capable of accommodating the different loading generated by the inter-axle differential during vehicle operation as well as overcoming the other above mentioned deficiencies with the prior art.